The Non-Dispersive Infrared (“NDIR”) technique has long been considered as one of the best methods for gas measurement. In addition to being highly specific, NDIR gas analyzers are also very sensitive, stable, reliable and easy to maintain. The major drawback of the NDIR gas measurement technique has been its relatively expensive implementation and high power consumption.
Ever since the NDIR technique of gas measurement was first introduced and practiced in the mid 1950's, a large number of improved measurement techniques based upon the NDIR principle for gas detection have been proposed and successfully demonstrated. The most notable advances over the years in this field are summarized as follows.
Burch et al. (U.S. Pat. No. 3,793,525) and Blau et al. (U.S. Pat. No. 3,811,776) in 1974 were the first to advance a so-called “Double Beam” technique for NDIR gas measurement by taking advantage of the principle of nonlinear absorption for some strongly absorbing gases such as CO2 to create a reference channel. Shortly thereafter, this “Double Beam” NDIR gas sensor technique was greatly simplified with the use of two interposed spectral filters (one absorbing and one neutral) to create a sample and a reference detector channel. Subsequent NDIR gas sensors, designed using this technique, have enjoyed good output stability as a function of time.
In U.S. Pat. No. 4,578,762 (1986) Wong advanced the first self-calibrating NDIR CO2 analyzer using a novel two-wheel chopper and mirror arrangement. Another improved type of such gas analyzer is shown and described in U.S. Pat. No. 4,694,173 (1987) by Wong. This gas analyzer has no moving parts for effecting the interposition of spectral filters or absorbing and non-absorbing cells to create both a sample and reference detector channel as in the NDIR gas analyzers described earlier.
In U.S. Pat. No. 5,163,332 (1992), Wong advanced the so-called “wave-guide” sample chamber for simplifying NDIR gas sensors into ones that are compact, rugged and low-cost while still maintaining their superior performance characteristics.
All of the NDIR gas analyzers described above for the measurement of the concentrations of one or more gases in a mixture perform well functionally and have contributed overwhelmingly to the overall technical advancement in the field of gas analysis during the past two decades. They have been widely accepted in both the medical and industrial communities. Despite their undisputed success over the years, there still remains an important application, namely the commonplace household fire detector, not successfully realized to date due to the fact that NDIR gas sensors are still too costly and consume too much power when used as sentinel fire detectors.
A majority of fire detectors in use today in almost all public buildings and private dwellings are in essence smoke detectors as they only detect the smoke resulting from a fire. The most common smoke detectors currently in use belong to two types. The first type is the so-called ionization smoke detector best for detecting invisible smoke particles ranging in size from <1.0 microns to ˜5 microns. The second type is called the photoelectric smoke detector best for detecting visible smoke particles >5 microns in size. For the past two decades, the ionization smoke detectors because of their low cost (<$10 retail) have dominated the fire market and are in use in over 90% of households. In recent years, photoelectric smoke detectors, because of their higher cost (<$30 retail), have fallen significantly behind in sales. Combined ionization and photoelectric smoke detectors, albeit at an even higher cost (˜$40 retail), have also been available for quite sometime but have not to date received much acceptance by the public.
Despite their low cost, relatively maintenance-free operation and wide acceptance by the buying public, the smoke detectors in widespread use today are not without problems and certainly are far from being ideal. One of the biggest problems with ionization smoke detectors besides being radioactive (Americium-241) is their frequent false-alarm. By the nature of its operational principle, any micron-size particulate matter other than smoke from an actual fire can set off the alarm. Kitchen grease particles generated by a hot stove is one classic example. Over-zealous dusting of objects and/or furniture near the detector is another. Frequent false-alarms are not just a harmless nuisance; some people actually disable their smoke detectors by temporarily removing the battery in order to escape such annoying episodes. This latter situation could be outright dangerous especially when these people forget to rearm their smoke detectors.
Another significant drawback for the current ionization smoke detector is its relatively slow speed to alert people of a fire. There are several factors that contribute to this particular drawback. The first fact is the detector trigger threshold for smoke which directly affects its response time to the onset of a fire. No doubt a lower trigger threshold would mean a faster fire detector. However, it also means more frequent annoying false alarms for the user. The second factor is the particular placement of the detector with respect to the spot where fire breaks out. Unlike ordinary gases, smoke is actually a complex sooty molecular cluster that consists mostly of carbon. It is much heavier than air and thus diffuses much slower than the gases we encounter everyday. Therefore, if the detector happens to be at some distance from the location of the fire, it will be awhile before enough smoke gets into the sampling chamber of the smoke detector to trigger the alarm. A third factor is the nature or type of the fire itself. Although smoke usually accompanies fire, the amount produced can vary significantly depending upon the composition of the material that catches fire. For example, oxygenated fuels such as ethyl alcohol and acetone give less smoke than the hydrocarbon from which they are derived. Thus, under free-burning conditions oxygenated fuels such as wood and polymethylmethacrylate give substantially less smoke than hydrocarbon polymers such as polyethylene and polystyrene. As a matter of fact, a small number of pure fuels, namely carbon monoxide, formaldehyde, metaldehyde, formic acid and methyl alcohol, burn with non-luminous flames and do not produce smoke at all.
Since fire is an oxidation process, detection of a sudden increase in ambient levels of one or more of the three principal effluent gases of fire, namely CO2, H2O and Carbon Monoxide (CO) is an effective way of detecting same. For the past 20 years, the use of CO2 sensor as a standalone fire detector or in combination with smoke detectors has been continually advocated by experts as the most effective fire detector. The reason is two-fold. First, there is a significant advantage of using a CO2 sensor rather than a smoke detector for fire initiation detection. The mobility of CO2 as a gas is far greater than that for smoke which is much heavier. Therefore CO2 diffuses from the fire to the detector in a much shorter time leading to a faster response time for enunciating fire. Second, over the past two decades, compact, low cost and reliable NDIR type CO2 sensors have become readily available. As a matter of fact, over the same period of time, a large number of deployment schemes, fire fighting techniques and fire control strategies, which use either a standalone NDIR CO2 sensor or in combination with smoke detectors, have been advanced. The most notable proposals of such are summarized as follows.
In U.S. Pat. No. 5,053,754 (1991), Wong advanced the first NDIR CO2 sensor used as a standalone fire detector. A fire detection system using at least two NDIR CO2 sensors positioned at spaced locations in an area for pin-pointing the exact origin of a fire was described in U.S. Pat. No. 5,079,422 (1992) by Wong. Meanwhile a standalone and compact low-cost fire detector which responds quickly to an increase in the concentration of CO2 gas in the ambient air was advanced in U.S. Pat. No. 5,103,096 (1992) by Wong. In U.S. Pat. No. 5,369,397 (1994), an adaptive fire detector taking advantage of the capability of an NDIR CO2 sensor for computing the rate of CO2 increase to shorten the response time for enunciating the onset of a fire was also advanced by Wong. In U.S. Pat. No. 5,592,147 (1997), an NDIR CO2 sensor used cooperatively in combination with a photoelectric smoke detector for significantly reducing false alarms was put forth by Wong. Also in 1997 and in U.S. Pat. No. 5,691,704, Wong disclosed another NDIR CO2 photoelectric smoke detector combination fire detector with special software which can be designed into a single semiconductor chip for cost reduction and further false alarm improvement. In U.S. Pat. No. 5,767,776 (1998), Wong disclosed the design of an NDIR CO2 and smoke detector combination which reduces the maximum average response time to less than 1.5 minutes. Further refinement of this design was described in U.S. Pat. No. 5,798,700 (1998) by Wong, U.S. Pat. No. 5,945,924 (1999) by Marman et al. and U.S. Pat. No. 5,966,077 (1999) by Wong. Finally, a method for dynamically adjusting criteria for detecting fire through smoke concentration using an NDIR CO2 and smoke detector combination was described by Wong in U.S. Pat. No. 6,107,925 (2000).
Despite the continual and persistent advocacy of many fire experts that an NDIR CO2 sensor, either as a standalone fire detector or in combination with a smoke detector, is better than present-day smoke detectors in both speed of response and proof against false alarms, it has yet to be exploited as a superior fire detector. The reasons are two-fold. First, even with the drastic cost reduction for present-day NDIR CO2 sensors, the cost is still far too high when compared with ionization type smoke detectors. Second and by far the most significant is the fact that being an NDIR gas sensor, its active infrared source uses far too much power when operated continuously. Because of this, it is not suitable for use in almost any circumstance, whether it is residential, commercial or industrial.